1. Field of the Invention
The present invention relates to management of a mobile network by mobile routers, where the mobile network is capable of changing its point of attachment to a wide area network (e.g., the Internet) and thus its reachability in its associated topology.
2. Description of the Related Art
Proposals have been made by Internet Engineering Task Force (IETF) groups for improved mobility support of Internet Protocol (IP) based mobile devices (e.g., laptops, IP phones, personal digital assistants, etc.) in an effort to provide continuous Internet Protocol (IP) based connectivity. For example, the IETF has a Mobile IP Working Group that has developed routing support to permit IP nodes (hosts and routers) using either IPv4 or IPv6 to seamlessly “roam” among IP subnetworks. In addition, the Mobile Networks (MONET) group (renamed as the Network Mobility (NEMO) group) has published different Internet Drafts, available on the World Wide Web at the Network Mobility Home Page at the address “nal.motlabs.com/monet” on the World Wide Web. One exemplary Internet Draft by Thierry Ernst, entitled “Network Mobility Support Terminology”, February 2002, is available on the World Wide Web at the address “nal.motlabs.com/monet/drafts/draft-ernst-monet-terminology-01.txt”.
For example, Ernst describes an exemplary mobile network that can be deployed within an airplane, where passengers establish an IP connection of their respective IP host devices (e.g., laptop, digital telephone, personal digital assistant, etc.) to a mobile router within the airplane for on-board Internet access; during the flight, the mobile router within the aircraft may change its point of attachment to the Internet via distinct Internet Service Providers (ISPs), for example by changing connections via respective radio links or geostationary satellite links for transoceanic flights. Note that a passenger also may have his or her own network (i.e., a personal area network) within the mobile network.
According to the NEMO group, a mobile network may be composed by one or more IP subnets and is connected to the global Internet via one or more Mobile Routers (MR). The mobile router has at least two network interfaces: an egress interface toward the wide area network, and an ingress interface from within the mobile network. Mobile network nodes may include local fixed nodes (LFN)(nodes unable to change their point of attachment while maintaining ongoing sessions), local mobile nodes (LMN)(mobile nodes that belong to the mobile network and able to change their point of attachment within the mobile network or outside the mobile network), and visiting mobile nodes (VMN)(mobile nodes that not belong to the mobile network and that can change their point of attachment from outside the mobile network to inside the mobile network). Each of the nodes may be either a host or a router.
Hence, a mobile router is a router configured for establishing a communication link between the mobile network and an attachment router of a wide area network, such as the Internet, providing connectivity for the mobile network to the wide area network. The mobile router thus serves as a gateway to route packets between the mobile network and the Internet.
The IETF also has a Mobile Ad-hoc Networks (MANET) Working Group that is working to develop standardized MANET routing specification(s) for adoption, by the IETF. According to the MANET Working Group, the“mobile ad hoc network” (MANET) is an autonomous system of mobile routers (and associated hosts) connected by wireless links—the union of which form an arbitrary graph. The routers are free to move randomly and organize themselves arbitrarily; thus, the network's wireless topology may change rapidly and unpredictably. Such a network may operate in a standalone fashion, or may be connected to the larger Internet.
The MANET system is particularly suited to low-power radio networks that may exhibit an unstable topology, where wireless propagation characteristics and signal quality between a wireless transmission source and a receiver can be difficult to model and quantify. In a MANET, the device address is tied to the device, not a topological location, as there is no fixed network infrastructure. When the addressed device moves, therefore, the motion changes the routing infrastructure. Hence, as described in an Internet Draft by Baker, entitled “An Outsider's View of MANET”, available from the IETF on the World Wide Web at the address “ietf.org/internet-drafts/draft-baker-manet-review-01.txt”, the fundamental behavior of a MANET is that a routing node carries with it an address or address prefix, and when it moves, it moves the actual address; when this happens, routing must be recalculated in accordance with the new topology. For example, each mobile router retains its address prefix; hence, neighboring mobile routers in a MANET may have distinct address prefixes.
A “Mobile IPv6” protocol is disclosed in an Internet Draft by Johnson et al., entitled “Mobility Support in IPv6”, available on the World Wide Web at the address “ietf.org/internet-drafts/draft-ietf-mobileip-ipv6-18.txt”. According to Johnson et al., the Mobile IPv6 protocol enables a mobile node to move from one link to another without changing the mobile node's IP address. Hence, a mobile node is always addressable by its “home address”, an IP address assigned to the mobile node within its home subnet prefix on its home link. Packets may be routed to the mobile node using this address regardless of the mobile node's current point of attachment to the Internet. The mobile node also may continue to communicate with other nodes (stationary or mobile) after moving to a new link. The movement of a mobile node away from its home link is thus transparent to transport and higher-layer protocols and applications.
A particular concern arises where a mobile network has a nested mobile network topology, where multiple mobile routers are recursively connected. FIG. 1A is a diagram illustrating a nested mobile network 10 having mobile routers 12a, 12b, 12c, 12d, 12e and 12f having nested attachments, enabling a local fixed node (e.g., LFN1) 14 to communicate with a correspondent node 16 via a wide area packet switched network, such as the Internet 18. Packets sent by the local fixed node 14 for the correspondent node 16 are received by the mobile router 12c; the mobile router 12c adds an IPv6 header having a source address specifying the home address of the mobile router 12c, and a destination address specifying the correspondent node 16. The path for the packets from the mobile router 12c to the Internet 18 is MR3 12c, to MR2 12b, to MR1 12a, to the Internet 18 via an access router 22.
As illustrated in FIG. 1A, the mobile router 12c has an associated home agent 20 for forwarding packets specifying the home address of the mobile router 12c as the destination address. The home agent 20, upon receiving the packets specifying the home address of the mobile router 12c, forwards the packets to the care of address registered for the mobile router 12c. the local fixed node 14. Although not shown, the mobile routers 12a and 12b would also have respective home agents.
FIG. 1B is a diagram illustrating bidirectional nested tunnels 22a, 22b, and 22c implemented by the mobile routers 12a, 12b and 12 for communications between the local fixed node 14 and the correspondent at 16. In particular, the Internet draft by Kniveton, T., entitled “Mobile Router Support with Mobile IP”, available on the on the World Wide Web at the address “ietf.org/internet-drafts/draft-kniveton-mobrtr-03.txt”, suggests a tunnel-within-tunnel approach as illustrated in FIG. 1B. Hence, according to Kniveton, communications between the local fixed node 14 and the correspondent node 16 would implemented by the mobile routers 12a, 12b, and 12c and the respective home agent 20a, 20b, and 20c based on bidirectional tunnels (e.g., 22a) between the mobile router (e.g., 12a) and the corresponding home agent (e.g. 20a).
The nested bidirectional tunnels 22a, 22b, and 22c may lead to very inefficient “pinball routing” in the infrastructure of the Internet 18, depending on the relative locations of the home agents 20a, 20b and 20c. 